The present invention relates to a small-dimension measurement system by scanning electron beam for measuring a small dimension of a sample such as a semiconductor device or the like by an electron beam.
Recently, there is a remarkable tendency of micronization of the processing dimensions of semiconductor devices and the measurement of the dimensions by light such as a laser beam or the like has already reached its limit. The development of measuring techniques using a high resolving power of an electron beam is needed for measurement of the pattern dimensions in the sub-micron region in association with realization of VLSIs. In many cases, a scanning electron microscopy is generally used for such measurement. However, the scanning electron microscopy is inherently an observing apparatus and differs from an small-dimension measurement system by scanning electron beam with respect to signal detecting and processing methods in that the latter system provides the signal with a high accurate position information which is required for measurement of small dimensions.
FIGS. 1A and 1B illustrate an example of a detected signal by one detector having a construction which is generally used in a conventional scanning electron microscopy (see T. E. EVERHART & R. F. M. THORNLEY, J. Sci. Instr., 37,246 (1960)). FIG. 1 is a diagrammatical cross section of a sample to be measured. A pattern 2 (in this example, design dimension L=1.2 .mu.m and a thickness is 0.8 .mu.m) is formed on a wafer substrate 1. FIG. 1B shows a signal waveform including position information to be obtained by deflecting a scanning electron beam over the pattern 2. It can be seen from FIG. 1B that there are differences between the peak values of and the signal widths .DELTA.u and .DELTA.w of the leading and trailing portions of the detected signal corresponding to edge portions 2u and 2w of the pattern 2. Although such signal waveforms at the edge portions may be influenced by the spot diameter of the scanning beam, fine configurations of the pattern edge portions, etc. the fact that the characteristics of the signal waveforms do not change even if the sample is rotated by 180.degree., shows that the above-mentioned signal waveforms are originated from the signal detection manner. This is because, in general, the scanning electron microscopy has only one detector and hence the pattern 2 itself to be measured causes a shadowing effect at the edge portion 2w. A shadow produced by this effect is effective to obtain far/near sense and unevenness sense of a scanned image of a sample in the scanning electron microscopy. However, the shadow provides a cause of measurement error from the viewpoint of dimension measurement by electron beam. In FIG. 1B, the length l of the shadow is approximately equal to the pattern dimension L and the leading and trailing widths of the signals detected at the edge portions 2u and 2w have a difference therebetween in order of 2-3 times. From such asymmetric detection signals, it is impossible to obtain the high-accurate measurement in the sub-micron region which is required in the small-dimension measurement by scanning electron beam.
There has been reported a system in which a plurality of detectors are used to improve the S/N ratio of signals (see Japanese Patent Application Laid-open No. 35854/83). But, the system does not take the above-mentioned shadowing effect into consideration and hence there is a drawback that the detection signals themselves include position information errors.